1. Field of the Invention
The invention relates generally to area navigation systems and more particularly to apparatus for controlling the vertical flight path of an airborne craft relative to preselected waypoint.
2. Description of the Prior Art
Automatic vertical navigation (VNAV) systems are relatively new to the art of area navigation systems. Nevertheless, problem areas in VNAV systems are analogous in some respects to some of the problem areas in lateral navigation systems (RNAV) and must be corrected or compensated for in order to provide a smooth flight on the vertical flight path. More specifically, such problems arise in attempting to provide a suitable and effective damping signal for the control system and in compensating for inherent discontinuities in the control data while the craft is in the vicinity of a VOR/TAC station.
For example, the vertical displacement signal (the signal representing the displacement or deviation of the craft from the computed vertical flight path) is typically subject to irregularities therein due to the method of computation. For example, a computerized system utilizing digital computation techniques will generally produce a vertical displacement signal with a stair/step or granular waveform in addition to the noise transients usually present rendering the signal unsuitable for deriving damping data therefrom.
Also, there may be an inherent discontinuity in vertical displacement signal which dramatically manifests itself when the craft passes over or near a VOR/TAC station. Specifically, the altitude of the craft is typically computed on the basis of the difference in barometric pressure between i) the craft and sea level for craft altitudes in excess of 18,000 feet; ii) the craft and the elevation of the VOR/TAC station for altitudes less than 18,000 feet. Because the reported station barometric pressure may be issued from a location some distance from the VOR/TAC station and because the pilot may err in setting the barometric pressure on the altimeter, an error of up to 2,000 feet may exist. As the craft traverses the vicinity of the VOR/TAC station (commonly referred to as passing through the "zone of confusion"), the slant range of the craft, which is accurately measured by a radio signal, will equal the altitude of the craft above the station. At this point, any error in the barometric altitude will become evident and will accordingly manifest itself in the vertical displacement signal. The system recognizes this problem and takes appropriate measures to prevent the craft from over-reacting to this error.